1. Field of the Invention
This invention relates to the field of hollow-core optical fibers, and in particular to the type of fiber known in the art as photonic crystal fiber (PCF), microstructured fiber or holey fiber.
2. Description of Related Art
Hollow core optical fiber waveguides, which are able to guide light by virtue of a so-called photonic bandgap (PBG), were first considered in 1995.
In, for example, “Full 2-D photonic bandgaps in silica/air structures”, Birks et al., Electronics Letters, 26 Oct. 1995, Vol. 31, No. 22, pp.1941-1942, it was proposed that a PBG may be created in an optical fiber by providing a dielectric cladding structure, which has a refractive index that varies periodically between high and low index regions, and a core defect in the cladding structure in the form of a hollow core. In the proposed cladding structure, periodicity was provided by an array of air holes that extended through a silica glass matrix material to provide a PBG structure through which certain wavelengths of light could not pass. It was proposed that light coupled into the hollow core defect would be unable to escape into the cladding due to the PBG and, thus, the light would remain localised in the core defect.
It was appreciated that light travelling through a hollow core defect, for example filled with air or even under vacuum, would suffer significantly less from undesirable effects, such as non-linearity and loss, compared with light travelling through a solid silica or doped silica fiber core. As such, it was appreciated that a PBG fiber may find application as a transmission fiber to transmit light between a transmitter and a receiver over extremely long distances, for example under the Atlantic Ocean, without undergoing signal amplification or regeneration, or as a high optical power delivery waveguide. In contrast, for standard index-guiding, single mode optical fiber, signal regeneration is typically required approximately every 50-80 kilometers.
The first PBG fibers that were attempted by the inventors had a periodic cladding structure formed by a triangular lattice of circular air holes embedded in a solid silica matrix surrounding a central air core defect. Such fibers were formed by stacking circular or hexagonal capillary tubes, incorporating a core defect into the cladding by omitting a single, central capillary of the stack, and then heating and drawing the stack, in a one or two step fiber-drawing process, to form a fiber having the required structure.
International patent application PCT/GB00/01249 (The Secretary of State for Defence, UK), filed on 21 Mar. 2000, proposed the first PBG fiber to have a so-called seven-cell core defect, surrounded by a cladding comprising a triangular lattice of air holes embedded in an all-silica matrix. The core defect was formed by omitting an inner capillary and, in addition, the six capillaries surrounding the inner capillary. This fiber structure was seen to guide one or two modes in the core defect, in contrast to the previous, single-cell core defect fiber, which appeared not to support any guided modes in the core defect.
A preferred fiber in PCT/GB00/01249 was described as having a core defect diameter of around 15 μm and an air-filling fraction (AFF)—that is, the proportion by volume of air in the cladding—of greater than 15% and, preferably, greater than 30%. Herein, AFF, or any equivalent measure, is intended to mean the fraction by volume of air in a microstructured, or holey, portion of the cladding, which is representative of a perfect, unbounded cladding. That is, imperfect regions of the cladding, for example near to or abutting the core defect and at the outer periphery of the microstructured region, would not be used in calculating the AFF. Likewise, a calculation of AFF does not take into account over-cladding or jacketing layers, which typically surround the microstructured region.
In the chapter entitled “Photonic Crystal Fibers: Effective Index and Band-Gap Guidance” from the book “Photonic Crystal and Light Localization in the 21st Century”, C. M. Soukoulis (ed.), ©2001 Kluwer Academic Publishers, the authors presented further analysis of PBG fibers based primarily on a seven-cell core defect fiber. The optical fiber was fabricated by stacking and drawing hexagonal silica capillary tubes. The authors suggested that there are many parameters that can have a considerable influence on the performance of bandgap fibers: choice of cladding lattice, lattice spacing, index filling fraction, choice of materials, size and shape of core defect, and structural uniformity along the length of the fiber.
In a Post-deadline paper presented at ECOC 2002, “Low Loss (13 dB) Air core defect Photonic Bandgap Fiber”, paper PD1.1, N. Venkataraman et al. reported a PBG fiber, having a seven-cell core defect, which exhibited loss as low as 13 dB/km at 1500 nm over a fiber length of one hundred meters. This loss level was, reportedly, an improvement of two orders of magnitude over previously published results. Although the fiber closely resembled the fiber described in the aforementioned book chapter, the authors of this paper attributed the reduced loss of the fiber as being due to the high degree of structural uniformity along the length of the fiber.
Charlene M. Smith et al describe in Nature, Vol. 424 (2003) at page 657 an air-guiding photonic crystal fiber exhibiting a loss of 30 dB/km between 1500 nm and 1700 nm, with a loss of 13 dB/km at 1500 nm. They identify coupling between the so-called surface and air-guided core modes of the fiber as an important contributor to transmission loss in a hollow core fiber. The surface modes are associated with the boundary of the air core and reside near to the silica/air interface. It was noted that reduced transmission occurred where the surface and core modes had the same propagation constant, at which point the spatial overlap provided an avenue by which the surface modes acted as intermediaries to couple light out of the core and into the quasi-continuum of lossy cladding and radiation modes.
The best commercially available, index-guiding telecommunications fiber, hereafter simply referred to as “standard fiber”, has a loss typically of the order of 0.16 dB/km. Thus, even the lowest loss levels of reported hollow-core PCF (hereafter referred to as “HC-PCF”) are still about two orders of magnitude higher than the loss levels of standard fiber. This degree of loss in HC-PCF clearly represents a problem in, for example, telecoms applications, which often involve propagation of pulses over very long distances. Loss levels can also be a significant consideration in fiber applications other than long-haul telecommunications.
It is generally recognized in the prior art that for a given design of HC-PCF it should be possible to reduce losses by careful fiber fabrication, with special care being taken to achieve structural homogeneity along the length of the fiber. We have found that pressurisation of some or all of the holes of the fiber, for example as described in our International Patent Application No. PCT/GB03/01298 (published as WO 03/080524), can be used to achieve improvements in this regard. We have also discovered that the size of the core hole and the shape and size of the region of material surrounding the core hole is significant in reducing loss. In UK patent application no. 0229826.3, we describe how increasing the size of the core hole can improve the amount of light that is guided in air of a HC-PCF. Also, in one example, described below and in our co-pending UK patent application nos. 0306593.5 and 0322024.1, the core boundary is substantially uniform in thickness and its thickness is such that the boundary region acts like a Fabry-Perot interferometer at anti-resonance, excluding light from the material of the boundary. A similar effect, described below and in our co-pending UK patent application nos. 0306606.5 and 0321991.2 can be achieved by providing a thinner core boundary but with thicker regions, or nodules, which are dimensioned to be antiresonant. By excluding light from the solid material forming the core boundary, the light propagates almost completely in the air-core of the fiber and in the surrounding holes. Theory predicts that exclusion of more than 99%, or even 99.9%, of the light from the glass may be attainable. Losses due to Rayleigh scattering of the bulk material of the fiber may, thereby, be greatly reduced.
So far, by careful fiber design, combined with systematic selection of appropriate fiber drawing parameters (for example, draw speed, draw tension, draw temperature and pressurisation of the core and (separately) the cladding holes) we have attained greatly-reduced losses, compared to the prior art; down to a level of around 2 dB/km over lengths of more than 1 km. We predict that we will achieve even lower losses, for example to a level of between 0.5 and 1.0 dB/km by producing fibers having an even higher degree of homogeneity.
However, even when comparatively good structural homogeneity is achieved, and light is substantially excluded from solid parts of the fiber, we have discovered that further loss-mechanisms will still undesirably limit the loss performance of HC-PCF.